Debt - Short-Term Debt (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Short-Term Debt
Short-term debt	$ 39.5	$ 51.4
Credit facilities	905.0	1,005.0
Outstanding borrowings related to credit facilities	39.5	51.4
Assets pledged as support for credit facilities	$ 0.0	$ 0.0
Weighted-average interest rates on short-term borrowings (as a percent)	4.70%	4.40%
Principal International Chile
Short-Term Debt
Credit facilities	$ 60.0
Principal Life Insurance Company
Short-Term Debt
Credit facilities	45.0
Line of credit
Short-Term Debt
Short-term debt	39.5	$ 51.4
Line of credit \| Principal Financial Services, Inc., Principal Life Insurance Company, and Principal Financial Group, co-borrowers
Short-Term Debt
Credit facilities	$ 600.0
Term of credit facility	5 years
Line of credit \| Principal Financial Services, Inc., Principal Life Insurance Company, Principal Financial Services V (UK) LTD, and Principal Financial Group, co-borrowers
Short-Term Debt
Credit facilities	$ 200.0
Term of credit facility	5 years
Credit facility maturing in March 2020	$ 11.0
Credit facility maturing in March 2022	$ 189.0
Commercial paper
Short-Term Debt
Support provided by back-stop facility for commercial paper program (as a percent)	100.00%	100.00%
Back-stop facility outstanding balances	$ 0.0	$ 0.0

Amount of assets pledged as support for credit facilities.

No definition available.

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- Definition

Amount of outstanding back-stop facilities as of the balance sheet date.

+ References

No definition available.

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- Definition

Percentage of support provided by back-stop facility for commercial paper program.

+ References

No definition available.

+ Details

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- Definition

The portion of the maximum borrowing capacity under the line of credit facility maturing in the fifth fiscal year following the latest fiscal year.

+ References

No definition available.

+ Details

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- Definition

The portion of the maximum borrowing capacity under the line of credit facility maturing in the third fiscal year following the latest fiscal year.

+ References

No definition available.

+ Details

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- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Weighted average interest rate of short-term debt outstanding calculated at point in time.

+ References

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- Details

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